Among conventional DC-DC converters which convert a DC voltage into another DC voltage, in particular, a current resonant type converter (of an LLC system or the like) has a circuit system basically easy to achieve high efficiency, which is one of typical circuits of an insulation type DC-DC converter and which allows for soft switching operation (see e.g. Patent Document 1).
A current resonant type converter recited in Patent Document 1 includes a switching circuit which has a switching element that is turned on or off in response to a control signal having a switching frequency and which converts a DC input voltage into an AC voltage by the switching element to output a first AC voltage; a resonance circuit which has resonant inductor and capacitor and receives an input of the first AC voltage to resonate at a predetermined resonance frequency, thereby outputting a resonance signal; a transformer having a primary wiring which receives an input of the resonance signal and a secondary wiring insulated from the primary wiring; a rectifier circuit which converts a second AC voltage output from the secondary wiring into a direct current to generate an output voltage and outputs the output voltage from an output side; and a control circuit which detects the output voltage and generates the control signal to turn on or off the switching element, the current resonant type converter having a predetermined output voltage-switching frequency characteristics at the switching frequency with respect to the output voltage, in which the control circuit includes an output voltage detection unit which detects the output voltage at starting to obtain a detection result, a frequency determination unit which determines the switching frequency for starting soft-start that enables power supply to the output side based on the output voltage-switching frequency characteristics and the detection result; and a frequency control unit which controls start of the current resonant type converter by turning the switching element on or off by the control signal that gradually decreases the switching frequency after starting at the determined switching frequency.
In such a current resonant type converter as recited in Patent Document 1, when applied to a solar panel or a storage battery in which an input or output voltage fluctuates largely, gain (output voltage/input voltage) control several times or wider is realized by decreasing resonant inductance values present in parallel on a preliminary side of a transformer of a circuit to increase a resonance current. However, in this case, losses caused by a resonance current are increased, resulting in a difficulty in achieving high efficiency.
Another system is proposed in which an additional circuit is provided for series compensation of a voltage fluctuation (see e.g. Non-Patent Document 1).
However, a bi-directional isolated DC-DC converter recited in Non-Patent Document 1 involves such problems as an increase in the number of switching elements and an increase in a loss in a compensation circuit.